This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This is a randomized, double-blind, placebo-controlled clinical trial which will evaluate the efficacy and safety of ALTU-135 treatment in CF patients with exocrine PI. It will be conducted as a multicenter study with participating sites having expertise in treating patients with CF. The study is divided into five phases: 1. Screening Period during which eligibility for study participation will be assessed. 2. Inpatient, off-enzyme Baseline -B Period during which each patients exocrine pancreatic function will be measured. 3. Open-Label Treatment Period during which all patients will receive ALTU-135. 4. Inpatient, Double-Blind DB Treatment Period during which half of the patients will be randomly withdrawn from treatment and receive placebo, and each patient?s exocrine pancreatic function will be measured for a second time. 5. Second Open-Label R Treatment Period during which all patients will resume treatment with open-label study drug. The total duration of treatment will not exceed 44 days and the total duration of the study excluding the Screening Visit will not exceed 64 days. Patients will be required to be inpatients for two, non-consecutive periods of approximately six to seven days.